Before 1995, most people believed that the only planets in the Universe were found in our Solar System. Since 1995, hundreds of “exoplanets,” or planets outside of our Solar System, have been discovered orbiting other stars. The Earth and our Solar System are not as unique as they were once thought to be. Planet formation is now considered to be very common in the Universe, and planets can form in the wake of the formation of any star. Star formation begins in giant gas clouds, and probably 99.9 percent of the material in these clouds goes into making the star. Only about 0.1 percent of the material in the original gas cloud is left for planet formation. This leftover material orbits the star and various forces cause the materials to begin crashing into one another. Over time, this process leads to the formation of very large objects—what we know as planets. Sometimes rocky like our Earth, sometimes gassy like Jupiter, these planets gather mass as other floating debris crashes onto their surfaces. In the early days of our Solar System, the Earth was constantly bombarded with floating debris. Over time, things settled down and the Earth cooled, making it the perfect place for life to form.
Scientists estimate that the Earth formed about 4.6 billion years ago. The Earth that we know today, the relatively peaceful home of myriad forms of life, didn’t appear overnight. Rather, it took billions of years to slowly evolve into its current form. The process of accretion that led to the formation of the Earth was a violent one, and it produced an Earth that was only a little less violent and hostile. For a few hundred million years, the early Earth was characterized by high temperatures, toxic gases, high levels of radiation, and ongoing high-impact collisions. Over time, these conditions improved and the Earth took on its distinctive structure with differentiated layers of core, mantle, crust, and atmosphere. This distinctive structure has important consequences: First, it helps explain why the surface of the Earth changes over time; and second, it helps explain why the Earth evolved into a suitable setting for living things.
In the last lesson, you learned that the interior of the Earth changed over time to give the planet the unique layering that it currently possesses. In this lesson, you will learn that the surface of the Earth changes as well. The high temperatures that caused the differentiation of the Earth’s layers continue even today. In a process known as plate tectonics, the heat and movement of the mantle floating under the Earth’s crust drive the movement of the Earth’s crust over its surface. The slow shifting of these tectonic plates explains the shape of our continents as well as mountain ranges and traumatic events like earthquakes.